1. Field of the Invention
The present invention relates to techniques for accommodating synchronous digital hierarchy (SDH) signals in an optical path network. It relates in particular to conversion between SDH signals transmitted at the electrical level and optical path signals transmitted at the optical level.
2. Description of Related Art
Wavelength division multiplexing (WDM) techniques, which make full use of the broadband nature of light, and optical path networks, which introduce wavelength routing into the path layer, have both been undergoing rapid development. It might be assumed that existing SDH, ATM and PDH (plesiochronous digital hierarchy) networks will all be accommodated in optical path networks. However, PDH networks are currently being replaced by SDH networks, and the majority of ATM networks are SDH-based systems which accommodate ATM cells in SDH paths. Therefore, the central task is the accommodation of SDH signals in optical path networks.
FIG. 1 illustrates a conventional switched network connecting subscriber terminals. Subscribers 11 and 18 are connected via switch 12, transit switch 13, line terminators 14 and 15, transit switch 16, and switch 17. Subscriber 11 and switch 12 are connected by, for example, a 64 kb/s channel, as are switch 17 and subscriber 18. Switch 12, transit switch 13 and line terminator 14 (and line terminator 15, transit switch 16 and switch 17) are connected by, for example, 52 Mbit/s paths comprising time division multiplexed 64 kb/s channels. Transmission between line terminators 14 and 15 involves further multiplexing of these paths.
FIG. 2 illustrates the connection of SDH signal paths via an SDH path cross-connect. Transit switches 21-1 to 21-4 are respectively connected to SDH path cross-connect 25 via line terminators 22-1 to 22-4 and 23-1 to 23-4. FIG. 2 shows a single path from transit switch 21-1 to each of transit switches 21-2 to 21-4, and in general transmission between line terminators 22-1 to 22-4 and 23-1 to 23-4 is carried out on the basis of time division multiplexed paths, with the direction of individual paths being set by SDH cross-connect 25.
FIG. 3 illustrates how some of the path connections shown in FIG. 2 are realized in an optical path network. In this case, optical path terminators 26-1 to 26-3 are used instead of line terminators, and these optical path terminators 26-1 to 26-3 are each connected to optical path cross-connect 27 via wavelength division multiplexed links. Optical paths can be established as desired between optical path terminators 26-1 to 26-3, and these optical paths are accommodated in the wavelength division multiplexed links using WDM techniques. A detailed account of such optical path networks is given in, for example, K. Sato and S. Okamoto, "Evolution of Path Layer Techniques Toward Photonic Networks", IEICE Japan Autumn Meeting, September 1992, SB-7-1, and K. Sato, S. Okamoto, and H. Hadama, "Optical Path Layer Technologies to enhance B-ISDN performance", Proc. IEEE ICC'93, June 1993, pp.1300-1307.
An optical path signal comprises a main signal and an optical path supervisory signal. Two types of signal are used as the supervisory signal. The first type is an optical path supervisory signal which is multiplexed at the electrical level in the same frequency band as the main signal. This type is primarily used for supervising the quality of the main signal. The second type is an optically added optical path supervisory signal which is multiplexed at the optical level in a different frequency band from the main signal or is superimposed onto the main signal by a modulation separate from that used for the main signal. This second type is primarily used for management and identification of the type of main signal. (See S. Okamoto, K. Oguchi and K. Sato, "Network architecture and management concepts for optical transport networks", Proceedings on IEEE/IFIP 1996 Network Operations and Management Symposium (NOMS '96), pp.1-11, April 1996, and Japanese Patent Application 8-49751, "Method for supervision of wavelength multiplexed optical communications" (not yet laid open at the time of filing of the present application)).
Thus, when an optical path signal is wavelength multiplexed and transmitted through WDM links, an optical path supervisory signal and a wavelength multiplexed optical path supervisory signal are transmitted along with it, these supervisory signals being contained in part of the optical path signal. The optical path supervisory signal and the wavelength multiplexed optical path supervisory signal are monitored, and if a fault occurs in an optical fiber or some transmission equipment, path restoration is performed by re-establishing the optical path along a route which detours around the location of the fault.
An explanation will now be given of the SDH transmission scheme. In SDH transmission, the unit of information transfer is the virtual container (VC), which comprises payload (information to be transmitted) and a "path overhead" which is added to this payload. When VC signals are to be accommodated in a transmission medium such as an optical fiber, a frame called a "synchronous transfer module" (hereinafter, STM) is formed. An administrative unit (AU) signal is formed by first adding pointers (AU pointers) to the VC signals in order to manage differences in the alignment of the VC signals with respect to the STM frame, and then time division multiplexing the VC signals. An STM-N signal is formed by time division multiplexing N of these AU signals and adding a section supervisory signal. After electrical to optical conversion of this STM-N signal has been accomplished, it is sent to the optical fibre. The reverse processing is carried out at a receiving terminal (see ITU-T Recommendation G.707, "Digital transmission system-Terminal equipments-General" and ITU-T Recommendation G.783, "Characteristics of synchronous digital hierarchy (SDH) equipment functional blocks"). When SDH signals are transmitted in wavelength multiplexed form, a plurality of STM-N signs are transmitted after being converted to optical signals of different wavelengths.
When a fault occurs somewhere in the transmission medium or in some transmission equipment, path restoration is performed by re-routing VCs around the location of the fault. It is stipulated that when a fault occurs, the termination equipment adjacent to the fault location maintains alignment with respect to the frame by generating STM-N or VC signals in which every bit is a logical "1" and using these as a substitute for the lost signal. In this case, because the SDH path supervisory signals in the VCs will not have valid values, faults can be detected in individual VCs and succeeding VCs re-routed.
The conventional signal format for accommodating an SDH signal in an optical path network will be explained with reference to FIG. 4 to FIG. 7. FIG. 4 shows the format of a VC signal, FIG. 5 the format of an STM-N signal, FIG. 6 the format of an AU signal, and FIG. 7 the format of an optical path signal. In these signal format diagrams, the direction of time elapse is from left to right along the horizontal axis. In addition to being read from left to right, the signals are also read vertically, so that reading starts at the top left of the frame and finishes at the bottom right (all subsequent drawings showing signal format will follow this pattern).
Different sizes of virtual container are defined, each capable of accommodating a different transmission rate. These different VCs are known as VC-11, VC-12, VC-2, VC-3, VC-4, VC-4-4c and VC-4-16c. FIG. 4 shows the format of a VC-4 signal. It will be seen that this comprises a 260.times.9 byte payload area P and a 9 byte path supervisory signal area PO. An STM-N signal comprises multiplexed VC signals of this sort. More precisely, as shown in FIG. 5, an STM-N signal has a 9.times.(261.times.N) byte payload area, a 3.times.(9.times.N) byte section supervisory signal area S1, a 5.times.(9.times.N) byte section supervisory signal area S2, and a 1.times.(9.times.N) byte AU pointer area a.
The signal format of a conventional optical path signal differs from that of an STM-N signal in the SDH transmission scheme. It is therefore necessary, when transferring an SDH signal through an optical communication network, to convert the SDH signal format to an optical path signal format.
Namely, when an STM-N signal is to be accommodated in an optical path, the signal has to be made longer and its transmission speed increased so that, as shown in FIG. 7, an optical path supervisory signal area can be provided and an optical path supervisory signal inserted therein.
There is also the problem that when a fault occurs within the optical communication network, two kinds of path restoration are required; namely, alternative routing of optical paths within the optical communication network, and alternative routing of VC signals in the SDH. As a result, increased network resources are needed.
The present invention has been devised in the light of this situation, and it is an object of this invention to provide optical path signal termination equipment capable of inserting an optical path supervisory signal without increasing signal length.